[syslinux] push-pop ordering error found in memdisk

[James Ling]

H. Peter Anvin,

The following problem came to my attention with memdisk. I cannot claim the
credit, but wanted to be sure the information made it back into the program
for the benefit of all users...

In file memdisk.asm, in respect of the operation of the raw and bigraw
options, the order of pop instructions does not correspond to the order
registers are pushed;

---------------------------------
;
; Routine to copy in/out of high memory
; esi = linear source address
; edi = linear target address
; ecx = 32-bit word count
;
; Assumes cs = ds = es
;
bcopy:
		push eax
		push ebx
		push edx
		push ebp

		test byte [ConfigFlags],CONFIG_RAW
		jz .anymode

		smsw ax			; Unprivileged!
		test al,01h
		jnz .protmode

.realmode:
		TRACER 'r'
		; We're in real mode, do it outselves

		pushfd	; <- *** push 1 ***
		push ds	; <- *** push 2 ***
		push es	; <- *** push 3 ***

		cli
		cld


---------------------------------
then a little further down;

		or dx,bx
		push dx	; Save A20 status ; <- *** push 4 ***

---------------------------------
but then after the copy, popping is in this order;

		pop es	; <- error pop order, *** pop 3 ***
		pop ds	; <- error pop order, *** pop 2 ***

		pop dx	; A20 status ; <- error pop order, *** pop 4 ***
		and dx,dx
		jnz .skip_a20d
		mov ax,2400h	; Disable A20
		int 15h
.skip_a20d:
		popfd		; <- *** pop 1 ***
		jmp .donev

---------------------------------

Best regards,

Jim


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